The invention relates to conductor track structures on a nonconductive support material, especially fine conductor path structures, and a process for production of the conductor track structures.
From the reprint, xe2x80x9cLAD-Ein neuartiges lasergestxc3xctztes Beschichtungsverfahren fxc3xcr Feinstleitermetallisierungenxe2x80x9d [a novel laser-assisted coating process for fine conductor metallization], from issue number 10, volume 81 (1990) of the technical periodical xe2x80x9cGalvanotechnik,xe2x80x9d it has become known to apply palladium acetate from a solution as a thin film to an entire surface in order to produce very fine conductor structures of well under 100 mm. By a subsequent laser treatment with an excimer laser with a wavelength of 248 nm, metal atoms are then said to be released in the area of the conductor track structures being produced, to serve as nuclei for a subsequent nonelectrical metallization. Prior to the metallization, however, it is necessary to perform a rinse to remove the undecomposed areas of the metal-containing film applied to the support material. The quality of this rinse plays a decisive part in the prevention of wild growth problems in the subsequent nonelectrical metallization. It has been found anyway that adequate adhesion of the deposited metal conductor tracks cannot be achieved by the described method.
It is the object of the invention to provide fine conductor track structures of electrical circuits that are simple and reliable to produce, especially also on three-dimensional circuit substrates, and furthermore to provide a substantially simplified and reliable method for producing conductor track structures, which will permit the production of fine structures by selective surface activation and reductive copper deposition.
This object is achieved by the invention as described hereinafter.
Since the heavy metal base of the support material in the area of the conductor track structures contains heavy metal nuclei which are formed by the breakup of an electrically nonconductive organic heavy metal complex applied to a microporous surface of the support material, a metallization can be performed without the necessity of first removing the untreated areas of the base containing the heavy metal in order to avoid wild growth problems.
Additionally, an outstanding adhesion of the deposited metal conductor tracks is achieved, since the support material contains microporous or microrough support particles to which the heavy metal nuclei are bound. In the metallization, a solid rooting is achieved by the copper growing into the pores and thus optimum adhesion of the applied conductor lines on the circuit support is assured.
The accessibility of the adhesion-promoting micropores is furthermore enhanced by the fact that the support material is comprised of a polymer matrix with embedded microporous or microrough support particles for the heavy metal nuclei which are exposed by the ablation of the polymer by the UV radiation applied to the surface, but are not themselves decomposed by the UV radiation. Thereafter, the workpiece can be directly metallized by chemical reduction. Thus, according to the invention, activation is performed with a double effect, in that on the one hand the micropores or the microroughness necessary for metal adhesion are exposed, and on the other hand the necessary heavy metal nuclei also are released there by the breakup of the organic, nonconductive heavy metal complex.
Since in the method of the invention an organic nonconductive heavy metal complex is bonded as a heavy metal containing component to microporous support particles, the support particles are mixed into the support material in the area of the conductor track structures that are to be produced, and/or they are applied and bound to the support material, an electromagnetic UV radiation is selectively applied in the area of the conductor track structures being produced, such that support particles are exposed by ablation and heavy metal nuclei are released by a breaking up of the bound heavy metal complex, and this area is then metallized by chemical reduction in order to form the conductor track structures, on the one hand the micropores or microroughness necessary for adhesion of the metal are exposed and on the other hand, even there the necessary heavy metal atoms are released by the breakup of the organic, nonconductive heavy metal complex.
It is an advantage that the chemically reductive metallization can be performed directly after the action of the electromagnetic UV irradiation. No problematic rinsing process is necessary. The UV radiation causes a breakup of the heavy metal complex in the area of the conductor track structures being produced, thereby splitting off heavy metal nuclei which are highly reactive for the partial reductive metallization. The metallization nevertheless takes place without any wild growth and very sharp contours are formed. The high reactivity of the heavy metal nuclei additionally favors the desired precise metallization in the necessary layer thickness.
Within the scope of the invention it is contemplated that electromagnetic radiation from a UV laser, an excimer laser or an UV radiator may be used. According to a preferred embodiment of the invention, a KrF excimer laser with a wavelength of 248 nm is used in order to expose the microporous filler particles and split off the heavy metal nuclei.
Preferably a palladium complex or a heavy metal complex containing palladium is used. As has been found, such heavy metal complexes are especially well suited for the very fine structuring by the method of the invention. In particular, ultraviolet radiation of a substantially lower energy density is sufficient for initiating the structuring cleavage reaction than is required for the ablation or also for triggering the action mechanism described as decomposition in known systems. In addition, substantially greater areas can be exposed per laser pulse in connection with the structuring than in known ablation techniques.
Within the scope of the invention, it is additionally envisioned that preferably a KrF excimer laser with a wavelength of 248 nm can be used for cleaving the heavy metal nuclei from the heavy metal complex. Thus it is possible to perform the cleavage without heating the complex. In this way melting of materials in the area of treatment is avoided. The result is a very great sharpness of the edges of the areas with cleaved heavy metal nuclei and, as a result, an extremely advantageous, very high edge sharpness in the metallized structures, which is of great importance, especially in very fine conductors.
According to a preferred embodiment, palladium diacetate is reacted with an organic complexing agent to form a palladium complex. As has been found, it is advantageous if a known, highly stable, polyfunctional chelating agent having several ligand atoms, such as N, O, S or P, is used as the organic complex forming agent. Within the scope of the invention it is furthermore provided that the polyfunctional chelating agent also can be used together with ionizing groups, such as hydroxyl or carboxyl groups.
In particular, molecular combinations of sterically hindered aromatic compounds and metal-complexing groups can be used as organic complexing agents. Preferably an organic complexing agent of the formula 
is used.
It is advantageous if support particles resistant to electromagnetic ultraviolet radiation are used as supports for the heavy metal complex. These are preferably inorganic-mineral support particles which are formed from pyrogenic silicic acid or from aerogels.
According to preferred embodiments of the invention the support particles are formed of pyrogenic silicic acid with a BET surface area of 200 m2/g or of aerogels.
Within the scope of the invention, it is furthermore contemplated that the binding of the heavy metal complex to the support particles is effected by immersing the particles in a solution of the heavy metal complex. The support particles prepared in this way are then mixed into the polymer material from which the circuit supports are injection molded. Alternatively, it is contemplated that the support particles with the heavy metal complex are mixed into a binder, especially a lacquer, and then applied as a coating to the support material.
The method of the invention can be used both with a surface application of laser radiation and masking techniques in an efficient mass production, and without masking, by means of guiding a point-focused laser beam under numerical control, for example, for making prototypes or small series production.